Several methods have been proposed and utilized for creating three-dimensional objects by the incremental material build up of thin layers. These processes include lamination, selective laser sintering, ballistic powder metallurgy, three-dimensional printing, stereolithography and near net thermal spraying. Lamination involves the simple process of cutting layers of a selected material and then bonding those layers together. The layers may be pre-cut to shapes corresponding to a cross section through the article to be created. Alternatively, standard shapes of material can be stacked and bonded together. Then, the assembled structure is cut or machined to produce the desired shape. In U.S. Pat. No. 4,752,352, Michael Feygin proposes a computer controlled method and apparatus for forming a laminated object. He provides a supply station, a work station for forming a material into a plurality of layers for lamination, an assembly station for stacking the layers in sequence into a three-dimensional object, a station for bonding the laminations to complete the formation of the three-dimensional object and a control station. In his patent, Mr. Feygin discloses a method in which the laminations are cut from a roll of material lifted, stacked and bonded under the direction of a computerized controller. The layers are bonded together by adhesive or brazing. This and other lamination techniques have several disadvantages. First, the bond between layers is critical and limits the strength of the object. Second, creation of each layer of the laminate also results in the production of significant amounts of waste materials. Third, the resulting object has a layered or serrated edge which must be removed by sanding or grinding. Finally, lamination is suitable for only those materials which can be formed into thin layers which can be bonded together.
In laser sintering, a laser is used to cure a starting material into a certain configuration according to the manner in which the laser is applied to that material. Stereolithography is a more recent yet similar process which creates plastic prototype models directly from a vat of liquid photocurable polymer by selectively solidifying it with a scanning laser beam. An example of this method is described in European Patent 322 257. Both of these methods require a substantial amount of curable raw material. In both cases the laser must be carefully controlled to achieve the desired shape. In some applications, the laser typically does not fully cure each cross section. Rather, the laser cures the boundary of a section and then cures an internal structure or honeycomb that traps the uncured fluid. Thereafter, the article must be subjected to final curing under separate ultraviolet lights or heat treatment. Additional post processing, such as careful sanding and grinding, is required for making smooth, accurate surfaces.
In ballistic powder metallurgy beams of particles are directed to the coordinates of a three-dimensional object in a three-dimensional coordinate system. A physical origination seed to which the particulate matter is attracted is required. The process may use a beam of particles directed to the origination seed which builds the particles upward from that seed. Alternatively, one can use an energy beam which attracts the particulate matter already in the environment to the seed or Masters in U.S. Pat. No. 4,665,492. This method cannot be used to make objects having undercuts therein without creating support structures at the same time. Normally, the support structures are created with the particle beam during the creation of the object. Such support structures must be removed by cutting, grinding or machining.
Three-dimensional printing is another technique similar to ballistic powder metallurgy. One variation of this technique creates layers of particles to produce a three-dimensional image in much the same manner that an ink jet printer produces two-dimensional images. The technique relies upon thermal shock or drop on demand material delivery techniques. A thermal shock technique forms a particle by vaporizing a small area of the fluid directly behind the nozzle. The drop on demand nozzle includes a piezo electric element to constrict the cavity thereby forcing a drop past the nozzle plate. In both instances the material is directed to a work surface in a manner to build up the article. This technique can only be used for certain kinds of materials.
In another variation of three-dimensional printing a series of two-dimensional layers are created by adding a layer of powder on top of a work surface. The powdered layer is selectively joined where the part is be formed by ink jet printing of a binder material. The work surface is then lowered and another layer of powder is spread out and selectively joined. The layering process is repeated until the part is completely printed. Following a heat treatment the unbonded powder is removed leaving the fabricated part. Although this technique has been proposed for metal, ceramic and plastic materials, it is limited to those materials to which a reliable binder can be applied.
None of the just described fabrication techniques have been successfully used to make steel parts. Of the previously described processes only laser sintering and ballistic powder metallurgy have been proposed for steel part fabrication. However, objects made from laser sintering are porous. Such porosity is not acceptable for most steel parts. Ballistic powder metallurgy utilizes a particle beam which is difficult to control to acceptable levels of accuracy.
The art has attempted to make objects by spraying layers of metal on a substrate. Problems have occurred in that the layers have tended to camber and possibly to peel apart from the substrate. Therefore, one must have a release agent or compatible substrate.
There is a need for a method and apparatus to manufacture quality metal parts by incremental build-up of material. The method and apparatus should be capable of producing articles having undercuts and irregular shapes.